The present invention relates to a system and method for controlling transmission power in a cellular mobile communications system for bidirectional communications.
In digital automobile telephone radio communications, bidirectional communications is carried out by setting radio channels between a base station and a mobile station, each having a transmitter and a receiver. In such communications, the transmission power must be suitably controlled to maintain the signal quality at a constant level.
JP-A-256331/1990 and JP-A-244056/1993 disclose the conventional prior art transmission control technique mentioned above.
In the configuration disclosed in JP-A-256331/1990, a base station includes level measuring means for measuring the receive level of a control signal transmitted from a mobile station and transmission means for transmitting level data judged by said level measuring means. A mobile station includes means for controlling the transmission level according to level data transmitted from the base station.
JP-A-244056/1993 discloses the configuration of controlling the transmission power according to an average receive bit error rate and a CIR (a carrier power to interference power ratio).
FIGS. 5 to 7 are explanatory diagrams each showing a prior art of obtaining a received signal level needed to control transmission power.
FIG. 5 is a diagram showing voice data encoded and transmitted for each frame and formed of plural slots. Each of slots forming each frame except the first slot is formed of a pilot (PL), a TPC (transmission Power Control) bit, and voice data. The first slot, or the leading portion of a frame, includes frame control data, instead of voice data.
The pilot in the first slot is formed of four symbols PL1, PL2, PL3, and PL4. Communication is performed under QPSK (Quadrature phase Shift Keying) modulation.
FIG. 6A and 6B are diagrams showing a diagram in which symbols PL1, PL2, PL3, and PL4 received are represented on an IQ plane. FIG. 6B is a diagram showing received signal levels obtained by calculating (squaring) the power of each symbol based on the received pilot symbols, and then calculating the average value of them. FIG. 7 is a diagram showing the configuration of a device, installed at a base station or mobile station, of creating a transmission power control command from a received pilot symbols.
As shown in FIG. 7, the prior art transmission power control device consists of a square unit 61, an averaging unit 62, and a comparator 63. The square unit 61 calculates the power of each symbol based on received pilot symbols. The average unit 62 averages the levels of power calculated by the square unit 61 to obtain a received signal level. The comparator 63 compares the received signal level with a reference value and then creates a transmission power control command which controls transmission power according to the comparison result.
In a base station and mobile stations, each having the above-mentioned configuration, the received signal level is periodically measured during a calling operation. The base station (or mobile station) compares the received signal level with a reference value. If the received signal level is more than the reference value, the base station (or mobile station) transmits a transmission power reducing command to a mobile station (or base station). When the mobile station (or base station) receives the command, it immediately decreases its transmission power by a predetermined control step width. When the received signal level is less than the reference value, the base station (or mobile station) transmits a transmission power increasing command to a mobile station (or base station). When the mobile station (or base station) receives the command, it immediately increases its transmission power by a predetermined control step width. Under such control, the received signal level can be maintained nearly to the reference value.
Now, let us explain how to obtain a CIR used for control. Conventionally, in order to obtain a CIR communications under PSK modulation, the power (full received power) of a signal converted into a base band signal is measured. Then the ratio of a desired signal level to an interference signal level is obtained. The desired signal level corresponds to the power of a signal after detection. The interference signal level corresponds to a level obtained by subtracting the desired signal level from the full received power.
In a power calculation, baseband signals are digitally sampled in symbol cycles. Then, the average value of the sampled signals regarded as a full received power. Similarly, a desired signal level is an average value of square values of signals after detection.
In the conventional transmission power control method in which a received signal level measured is compared with a reference value, there is the disadvantage in that when an actually-received signal level is measured with poor precision, it deviates from a target value, whereby transmission power varies extremely.
Moreover, there is the disadvantage in that fading particularly degrades the precision in level measurement because of large influence of noises, except desired waves.
When the transmission power is controlled using a CIR, interference causes a large error in the full received power and desired signal level obtained through the above-mentioned calculation, thus occurring troubles in transmission power control.